This invention relates to improved electrochemical cells or batteries and more particularly to an electrochemical cell having a novel type of material for use as a current collecting substrate for supporting the active material in a pasted or pressed plate construction.
A cell plate or electrode in an electrochemical cell is comprised of a substrate of thin sheet-like material with a layer of active material on each side. In a so-called "pasted" plate construction, the active material may be spread onto the substrate or applied by other well-known techniques. Usually the active material in paste form is spread with a doctor blade or by calendering so that the paste is uniformly distributed over area of the substrate which is to be electrochemically active in charge/discharge cycles of the cell.
Expanded metal and woven screen or mesh of electrically conductive material are examples of known substrate materials for pasted plates. The choice between the different substrate constructions has been guided by the fact that the active material composition has a tendency to dislodge from the substrate due to vibration occurring during processing or use. Also, after extended use, volumetric changes during charge/discharge cycles have a tendency to cause the composition to dislodge. For this reason screen wire mesh and expanded metal materials for substrate construction have been almost uniformly adopted in the industry for pressed plate construction.
One commercially available form of these materials is a wide sheet configuration. However, since most cell plates have at least one narrow dimension, the wide sheets must be cut into strips resulting in a raw edge portion defined by the sharp endsof the severed screen wires or expanded metal segments. These constructions have proved to be unsatisfactory because the raw edge complicates further processing of the cell, particularly by penetrating the separator and causing an electrical short between the cell plates.
To date, there is no known solution to the problems caused by the raw edge construction when using expanded metal materials.
With respect to the screen wire construction, these problems have been overcome by special manufacture of the screen in narrow strips with a selvaged edge, i.e., the wires running parallel to the narrow dimension are woven from one continuous strand such that the edge of the strip is defined by looped portions of the single strand. This construction adds greatly to the cost of the screen. Furthermore, the screen mesh, if small diameter wire is used, is easily broken which can cause shorting between the plates, an open circuit, or an increase in impedance. Also, the small diameter of the wires inherently yields a substrate of relatively high impedance.